<Chapter Label = "Attributes and operations for semigroups">
<Heading>
Attributes and operations for semigroups
</Heading>
In this chapter we describe the methods that are available in &SEMIGROUPS; for
determining the attributes of a semigroup, and the operations which can be
applied to a semigroup.
<Heading>
Expressing semigroup elements as words in generators
</Heading>
It is possible to express an element of a semigroup as a word in the
generators of that semigroup. This section describes how to accomplish this
in &SEMIGROUPS;.<P/>
The &SEMIGROUPS; package provides methods to calculate the maximal
subsemigroups of a finite semigroup, subject to various conditions. A
<E>maximal subsemigroup</E> of a semigroup is a proper subsemigroup
that is contained in no other proper subsemigroup of the semigroup.
<P/>
When computing the maximal subsemigroups of a regular Rees (0-)matrix
semigroup over a group, additional functionality is available. As described
in <Cite Key="Graham1968aa"/>, a maximal subsemigroup of a finite regular
Rees (0-)matrix semigroup over a group is one of 6 possible types. Using the
&SEMIGROUPS; package, it is possible to search for only those maximal
subsemigroups of certain types.
<P/>
A maximal subsemigroup of such a Rees (0-)matrix semigroup <C>R</C> over a
group <C>G</C> is either:
<Enum>
<Item><C>{0};</C></Item>
<Item>formed by removing <C>0</C>;</Item>
<Item>formed by removing a column (a non-zero &L;-class);</Item>
<Item>formed by removing a row (a non-zero &R;-class);</Item>
<Item>formed by removing a set of both rows and columns;</Item>
<Item>isomorphic to a Rees (0-)matrix semigroup of the same dimensions over
a maximal subgroup of <C>G</C> (in particular, the maximal subsemigroup
intersects every &H;-class of <C>R</C>).</Item>
</Enum>
Note that if <C>R</C> is a Rees matrix semigroup then it has no
maximal subsemigroups of types 1, 2, or 5. Only types 3, 4, and 6
are relevant to a Rees matrix semigroup.
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